A First Order Phase Transition and Self-organizing States in Single-domain Ferromagnet

Analytical consideration of uni-axial single-domain ferromagnet during the first order phase transition induced by a magnetic field is performed. Field is directed along the symmetry axis antiparallel to initial magnetization direction. For samples of the flat shape, besides the known change of the magnetization direction on 180º, at definite relations between values of magnetic field, the magnetization and the anisotropy of a crystall, there is continuous spectrum of states with intermediate magnetization directions. In these states, a precession frequency ω = 0. For samples of spherical shape, a process of the phase transition does not depend on the demagnetization field. At addition action of high frequency field perpendicular to the main magnetic field, there are dynamic equilibrium states, i.e. "self-organizing states" of ferromagnet, when the entropy increase connected with dissipation is compensated by the negative entropy flow due to the periodic field. It is shown that under these conditions, by varying the frequency of the periodic field, we can control the self-organising system, i.e. decrease or increase the system energy and, correspondingly, change the direction of magnetisation in ferromagnet.